Photometric interference determination

A method of determining a level of interference with a photometric in-vitro diagnostic assay, the method comprising

• treating an aliquot of a sample with at least one reagent to obtain a sample/reagent mixture and subjecting the sample/reagent mixture to a photometric measurement in order to obtain a result of the in-vitro diagnostic assay, and during the same photometric measurement determining a preliminary level of interference by semi-quantitatively determining one or more interfering substances in the same sample/reagent mixture,
• wherein determining the preliminary level of interference comprises extrapolating photometric measurement values obtained during the in-vitro diagnostic assay to an initial time of sample/reagent mixture formation or start of reaction or taking an average of measured data points closest to the initial time, and
• only upon determining a preliminary level of interference above a predetermined threshold triggering a separate photometric measurement of another aliquot of the same sample either undiluted or diluted with a liquid other than a reagent in order to determine an effective level of interference by quantitatively determining the one or more interfering substances.